A novel exopolysaccharide-containing structure has recently been discovered that localizes to a single pole of Agrobacterium tumefaciens cells and appears to promote cellular contact with surfaces, presumptively functioning as an adhesin. Production or extrusion of the exopolysaccharide is stimulated under growth conditions with limiting phosphorous and is regulated by the PhoR-PhoB two component system. A. tumefaciens is a pathogenic microbe that transfers DNA and proteins directly to plant host cells via a Type IV Secretion (T4S) system. There is virtually nothing known regarding the mechanism by which A. tumefaciens attaches to plant cells during the infection process. Unlike mammalian pathogens, there is limited understanding of host cell surface interactions in plant pathogens. The wide potential host range of A. tumefaciens, including fungi and mammalian cells, as well as its proclivity to form biofilms on diverse surfaces suggest an extremely versatile adhesion mechanism. It is well documented that A. tumefaciens tightly attaches to plant tissues and abiotic surfaces in a polar orientation. Recent evidence suggests that the A. tumefaciens T4S is also localized to a single pole of the cell, analogous to the polar exopolysaccharide. This study will determine whether the exopolysaccharide and the T4S machine can occupy the same pole of the cell, whether this occurs during surface colonization, and if the exopolysaccharide plays a role in productive infections. Co-localization of additional polar structures, flagella and pili, will also be tested. The chemical composition of the exopolysaccharide will be determined and the genes encoding its synthesis, elaboration and localization isolated. The mechanism of phosphorous-responsive regulation of the exopolysaccharide will be examined and the integration of this structure with other adhesion pathways during adaptation to the surface environment elucidated. The findings generated in this study will shed light on the process of surface attachment and biofilm formation during plant host interactions, including implementation of the T4S toxin delivery system. The properties of this polar polysaccharide are similar to the adhesive holdfast of Caulobacter species and suggest that these structures could be more common than anticipated, and may provide novel antimicrobial targets. The pathogenic microbe Agrobacterium tumefaciens produces a sticky, sugar-containing substance that concentrates on one end of cell. We are investigating the role of this structure in disease and bacterial survival using molecular genetics, high resolution microscopy and genomic analysis. This study promises to provide important insights into the relationship between adhesion and disease progression.